Pulmonary function tests are confusing to many COPD patients, and understanding how your doctor interprets the results can be even more baffling. However, knowing your numbers as they apply to pulmonary function tests is just as important as knowing your blood pressure if you have heart disease. Not only do your test results give you a method of comparison when determining how well you're responding to COPD treatment, but they also tell you if your disease is improving, staying the same, or progressing.
Purpose of Pulmonary Function Tests
In the diagnosis of COPD, pulmonary function tests are performed to assess lung function and determine the degree of damage to the lungs. Along with a patient's history and physical, lung imaging studies (chest X-ray, CT scan) and open lung biopsy, PFTs are critically important in the evaluation of respiratory health.
Pulmonary function tests are used when:
- Screening for the presence of obstructive and/or restrictive lung diseases.
- Determining the patient's condition prior to surgery in order to assess the risk of respiratory complications after surgery.
- Evaluating the ability of a patient to be weaned off a ventilator.
- Assessing the progression of lung disease and the effectiveness of treatment.
COPD causes the air in the lungs to be exhaled at a slower rate and in smaller amounts compared to a person with healthy lungs. The amount of air in the lungs isn't readily exhaled due to either a physical obstruction (such as with mucus production) or airway narrowing caused by chronic inflammation. That's where spirometry comes in.
How is the Test Performed?
Spirometry is the most common of all the pulmonary function tests. It is performed with a hand-held device called a spirometer and can easily be used by patients with the assistance of an experienced technician. It is normally the clinician's first choice when attempting to diagnose a respiratory problem. It is also commonly used to define the grades (stages) of COPD. A convenient, noninvasive procedure, spirometry can be performed in the privacy of your doctor's office or at most inpatient or outpatient facilities.
The test requires the patient, after all air has been expelled, to inhale deeply. This maneuver is then followed by a rapid exhalation so that all the air is exhausted from the lungs. Spirometry test results vary, but are based on predicted values of a standardized, healthy population (see below).
Common Terminology Used in Spirometry Testing
Spirometry tests measure the rate of change in lung volumes during forced breathing maneuvers. Those measurements are expressed using the following terminology:
- Vital Capacity (VC) — The amount of air that can be forcibly exhaled from your lungs after a full inhalation.
- Forced Vital Capacity (FVC) — The amount of air which can be forcibly exhaled from the lungs after taking the deepest breath possible.
- Forced Expiratory Volume in One Second (FEV1) — The amount of air which can be forcibly exhaled from the lungs in the first second of a forced exhalation.
- FEV1/FVC Ratio — Also expressed as FEV1% or %FEV1. It gives the ratio of FEV1 to FVC telling the clinician what percentage of the total amount of air is exhaled from the lungs during the first second of forced exhalation.
- Peak Expiratory Flow Rate (PEFR) — Measures if treatment is effective in improving airway diseases such as COPD.
- Forced Expiratory Flow (FEF) — A measure of how much air can be exhaled from the lungs. It is an indicator of large airway obstruction.
- FEF25% — This value represents the amount of air that can be forcibly exhaled from the lungs in the first 25% of the total forced vital capacity test.
- FEF50% — This measurement represents the total amount of air expelled from the lungs during the first half (50%) of the forced vital capacity test. It is useful when the clinician suspects an obstructive disease. In a patient with an obstruction, the amount of air that will have been exhaled will be less than that of someone with healthy lungs.
- FEF25%-75% — This measurement is representative of the total amount of air exhaled from the lungs during the middle half of the forced vital capacity test. Many clinicians refer to this value because it't indicative of an obstructive lung disease.
- Maximal Voluntary Ventilation (MVV) — A value determined by having the patient inhale and exhale as rapidly and fully as possible in 12 seconds. The results reflect the status of the muscles used for breathing, how stiff the lungs are and if there is any resistance in the airways when breathing. This test tells surgeons how strong a patient's lungs are prior to surgery. If patients demonstrate poor performance on this test, it suggests to the doctor that respiratory complications may occur after surgery.
Interpreting Your Spirometry Test
Spirometry helps doctors identify the presence of obstructive and restrictive lung diseases. Like any diagnostic test, your doctor is the best person with whom you should discuss your spirometry test results and the only one who can provide you with an accurate diagnosis.
There are a number of methods used for interpretation; the one your doctor uses is a matter of preference. What's truly important in any spirometry test is that it is done correctly, and that the test is interpreted accurately and systematically by a trained clinician.
The following is just one method of test interpretation that your doctor may use. It is meant for informational purposes only and should not replace sound medical advice from your health care provider:
- Step 1: First, look at your FVC to see if it's normal.
- Step 2: Next, look at your FEV1 to see if it's normal.
- Step 3: If both your FVC and your FEV1 are normal, you have a normal test and you can stop at this step.
- Step 4: If your FVC and/or your FEV1 are low, then your may have a lung disease and further tests are needed to evaluate this.
- Step 5: Finally, if Step 4 suggests a possibility of lung disease, look at the %predicted for your FEV1/FVC. If it is 69% or less, an obstructive disease should be suspected. If it is 85% or higher, then a restrictive lung disease should be suspected.
How Do Doctors Determine the Severity of COPD?
Doctors also use spirometry to determine the severity of COPD. Although there are a number of systems to choose from, the table at the bottom of this page is the method recommended by the Global Initiative for Obstructive Lung Disease (GOLD).
Your test results are compared with tables of normal, predicted values that use demographic variables such as age, gender, body size and ethnicity as a method of standardization. The percent predicted, as the term is referred to, gives clinicians a tool for comparing your test results with those results obtained from people with healthy lungs.
Spirometry test results will usually be measured twice — both before and after the administration of a bronchodilator. According to the American Thoracic Society, if there is at least a 12% and 200 milliliter improvement from baseline in post-bronchodilator FEV1 in two out of three spirometry measurements, you are said to respond significantly to a bronchodilator, which should correlate with how well you respond to treatment, an important prognostic factor.
Factors That May Influence Your Spirometry Results
Oftentimes, people undergo spirometry and then question if the results are truly accurate. Indeed, there are certain factors that can influence your spirometry results in a less-than-favorable way. To find out more about what these factors are, read:
What Other Tests Are Important to the Diagnosis of Lung Disease?
Along with spirometry, two other pulmonary function tests are important in the diagnosis of lung disease:
Diffusion Studies — This PFT tells you how well the oxygen that you breath moves into your bloodstream.
Body Plethysmography — A test which determines how much air is present in your lungs when you take a deep breath and how much air is left in your lungs after you exhale as much as you can.
Still unsure about your PFT results? Talk to your health care provider or visit the COPD Forum to ask your Guide, Deborah Leader RN, a question.
GOLD Spirometric Criteria for COPD Severity
|GOLD Spirometric Criteria for COPD Severity|
|I. Mild COPD||* FEV1/FVC < 0.7 |
* FEV1 >/= 80% predicted
|At this stage, the patient is probably unaware that lung function is starting to decline|
|II. Moderate COPD||* FEV1/FVC < 0.7 |
* 50% </= FEV1 < 80% predicted
|Symptoms during this stage progress, with shortness of breath developing upon exertion.|
|III. Severe COPD||* FEV1/FVC < 0.7 |
* 30% </= FEV1 < 50% predicted
|Shortness of breath becomes worse at this stage and COPD exacerbations are common.|
|IV. Very Severe COPD||* FEV1/FVC < 0.7 |
* FEV1 < 30% predicted or FEV1 < 50% predicted with chronic respiratory failure
|Quality of life at this stage is gravely impaired. COPD exacerbation can be life threatening.|
Northern Arizona University. Pulmonary Function Testing.
Global Strategy for the Diagnosis, Management and Prevention of COPD, Global Initiative for Chronic Obstructive Lung Disease (GOLD) 2010. Available from: http://www.goldcopd.org.